“Shoulders” on the high‐latitude magnetopause: Polar/GOES observations

MHD simulations predict that under strongly southward interplanetary magnetic field (IMF) conditions the polar cap potential saturates and attendant Region 1 currents affect the dayside magnetosphere by weakening subcusp magnetic fields and enhancing them in the high‐latitude lobe. Consequently, “shoulder‐like” structures appear as sunward projections of the high‐latitude magnetopause relative to the location of the subsolar magnetopause that retreats earthward. We present observations from the Polar satellite, acquired during a period of strong southward IMF and saturated transpolar potential, which illustrate the predicted lobe field enhancement and resulting “shoulder” structure. At about the same time the GOES satellite observed weakening magnetic fields near the subsolar magnetopause. Similar types of observations, obtained under weakly and strongly driven magnetospheric conditions, are shown to be consistent with the MHD predictions. We demonstrate that including the Region 1 current generated magnetic field in the force balance equation at the magnetopause provides a quantitative basis for predicting magnetopause erosion during strongly southward IMF intervals.